When try_add_var is looking for values that can be moved out of the way
in order to satisfy constraints for the current instruction, avoid
values that are live on a CFG edge originating at the current (branch)
instruction.
These values must be in their globally assigned location when entering
the branch destination EBB.
This is covered by the existing regalloc/iterate.cton test case which
fails with an upcoming commit.
12a8d6cce1